Read the new white paper "Authentication in Firebird" from IBPhoenix: Developments introduced in Firebird 3 provide many options for authenticating users attempting to attach to a database. No longer is authentication restricted to a single method: the methods available are determined by the plug-in authentication modules configured in the global and/or database config files.
Also, there are many interesting articles in Documentation area of IBPhoenix website.

We are glad to announce the new Firebird Forum for German-speaking developers: www.firebirdforum.de, with moderator Martin Koeditz, the editor of German edition of Firebird Language Reference. Fill free to register and meet with other Firebird developers! 

"Firebird Referenz Dokumentation" (in German), the translation made by Martin Köditz and Mark Harris, is released. Many thanks for the translators for their hard work!
It is available on the main Firebird Documentation page.